Density excitations of a harmonically trapped ideal gas

The dynamic structure factor S(q, ω) of a harmonically trapped Bose gas has been calculated well above the Bose-Einstein condensation temperature by treating the gas cloud as a canonical ensemble of non-interacting classical particles. The static structure factor is found to vanish s8 q ² in the long-wavelength limit. We also incorporate a relaxation mechanism phenomenologically by including a stochastic friction force to study S(q, ω). A significant temperature dependence of the density fluctuation spectra is found. The Debye-Waller factor has been calculated for the trapped thermal cloud as a function of q and the number N of atoms. A substantial difference is found for small- and large-N clouds.     

Homogenization of the layered medium equation

A new partial differential equation to be called the layered medium equation is introduced, and it is proved that certain relevant initial or periodic boundary conditions give well-posed problems. Then, the homogenized limit of the layered medium equation is studied. It is shown to be preserved in limit in the limit in the physical problem in which the coefficients that arise from the dielectric layer are both proportional to thickness. Otherwise, a non-local problem is obtained as the limiting form     

Ebullioskopie

Ohne Zusammenfassung     

Spin wave resonance study of exchange-coupled Ni80Fe20 double layers separated by thin interlayers of Cu, Cr and Pd

We have performed ferromagnetic resonance measurements on thin polycrystalline Ni₈₀Fe₂₀ (permalloy) double layers with interlayers of Cu, Cr and Pd. Furthermore, a mathematical model is presented, which allows a calculation of resonance fields in the parallel configuration. Comparison of the calculated and measured resonance field values yields an interlayer exchange coupling constant A₁₂ as a function of interlayer thickness d. We observe exponential dependencies of A₁₂ on d for Cr, Cu and Pd interlayers with the strongest decay for Cr. Additionally, we present FMR measurements and calculations for a Ni₈₀Fe₂₀/Cr/Co heterostructure layer system.     

Positron lifetime and Doppler-broadening measurements in noble metals and their alloys

Positron annihilation measurements in pure copper and silver as well as in the alloysCuGe,CuIn,CuSb, andAgSb have been performed. Most of the measurements have been carried out in thermal equilibrium. In this way it is possible to deduce vacancy formation — and vacancy — solute atom binding energies. ForCuIn alloys with 3.3 at.% and 7.1 at.% indium precipitation effects have been studied.This paper is based upon a talk given by W.L.-T. at the Int. Symp. Production of Low-Energy Positrons with Accelerators and Applications Justus-Liebig-Universität Giessen (1986)     

Dynamics of uniform quantum gases, II: Magnetic susceptibility

A general expression for temperature-dependent magnetic susceptibility of quantum gases composed of particles possessing both charge and spin degrees of freedom has been obtained within the framework of the generalized random phase approximation. The conditions for the existence of dia-, para-, and ferro-magnetism have been analyzed in terms of a parameter involving single-particle charge and spin. The limit T→0 retrieves the expressions for the Landau and the Pauli susceptibilities for an electron gas. It is found for a Bose gas that on decreasing the temperature, it passes either through a diamagnetic incomplete Meissner-effect regime or through a paramagnetic-ferromagnetic large magnetization fluctuation regime before going to the Meissner phase at .     

Defects and their removal in block copolymer thin film simulations

In recent years, there has been increased interest in using microphase-separated block copolymer thin films as submicrometer/suboptical masks in next generation semiconductor and magnetic media fabrication. With the goals of removing metastable defects in block copolymer thin film simulations and potentially examining equilibrium defect populations, we report on two new numerical techniques that can be used in field-theoretic computer simulations: (1) a spectral amplitude filter (SF) that encourages the simulation to relax into high symmetry states (representing zero defect states), and (2) different variants of force-biased, partial saddle point Monte Carlo algorithms that allow for barrier crossing toward lower energy defect-free states. Beyond their use for removing defects, the force-biased Monte Carlo algorithms will be seen to provide a promising tool for studying equilibrium defect populations. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2495–2511, 2006